Definition: You are given a question on conceptual physics. You are also given 4 answer options (associated with "A", "B", "C", "D"), out of which only one is correct. You need to answer the question by selecting the correct option. You should only answer with the choice letter, not the whole answer.
Input: Most of the radiation in Earth’s biosphere is
(A)natural background radiation (B)the result of military activities (C)from nuclear power plants (D)in the form of cosmic rays

A